In the late 1940s the quadrilateral socket was introduced to the United States, and during the intervening years since its introduction the "quad socket" has been the standard design for fitting a prosthetic limb to the residual thigh of an above-knee amputee. During the more than 30 years in which the quad socket has been employed in the United States it has become clear, however, that there are certain problems, particularly as to comfort and stability, inherent to the use of the quad socket.
Although there are many variations to the quad socket, it consistently presents a horizontally oriented brim at the proximal posterior. The horizontal brim serves as the ischial seat upon which the user's ischial tuberosity is supported. In fact, that portion of the user's weight which would normally have been supported by the amputated limb is transferred to the quad socket through the ischial seat.
The geometry of the quad socket purposely provides a rather narrow dimension, measured between the anterior and posterior walls of the socket, in relation to the medial/lateral dimension. The aforesaid dimensional relationship of the quad socket was selected to assure that the socket applies pressure on the anterior of the residual thigh to push the ischium toward the posterior of the socket in order that the ischial tuberosity will be forced to rest solidly on the ischial seat presented by the brim at the proximal posterior of the socket. To accommodate the compression of the thigh which results within the quad socket because of the purposely narrowed anterior/posterior dimension, the medial/lateral dimension of the socket is made relatively larger.
However, the enlarged medial/lateral dimension makes it virtually impossible to provide sufficient lateral support for the distal end of the femur in the residual limb, as would be required to eliminate a limp during that portion of the user's gait when the artificial limb is in the weight bearing mode and the sound leg is swinging through to the next step. In fact, many knowledgeable commentators are of the opinion that the quad socket is ineffective in all but the mid-stance phase of the gait due to the excessive abduction of the femur permitted within the quad socket because of the aforesaid dimensional relationship.
Recognition of the deficiencies inherent to the quad socket is, in effect, a challenge to the basic concept that the ischial tuberosity should serve to transmit the majority of the weight expected to be carried by the residual limb to the ischial seat provided for that purpose on the quad socket. Such a challenge recognizes that the structural arrangement of the quad socket has the basic inability to stabilize the femur when the gluteus medius fires; the arrangement of the quad socket, after all, provides no structure whereby the ischium is able to preclude abduction of the femur. It is this inability to stabilize the femur which results in the necessity for the user to lean laterally in an attempt to stabilize the pelvis, thus presenting the readily recognizable, and characteristic, limp required of a person using the quad socket.
Specifically, as the gluteus medius pulls the femur into abduction, the pelvis slides medially because the ischial tuberosity is free to shift along the ischial seat of the quad socket; The unsupported femur has little choice but to abduct in a more pronounced attitude within the wide medial/lateral dimension of the quad socket. The pronounced abduction imposes pain at the distal end of the femur as well as at the proximal medial portion thereof. To reduce the undesirable pressure, and the resulting pain, the patient leans to position the torso over the abducted, distal end of the femur.
The aforesaid, negative characteristics of the quad socket tend to be obviated by a much more recent innovation in prosthetic sockets which employs a narrower medial/lateral dimension and a wider anterior/posterior dimension, the relative dimensions being chosen such that the ischial tuberosity and a portion of the ramus of the ischium is ostensibly to be contained within the socket. The recently developed, narrow medial/lateral socket configuration also employs a relatively high lateral wall which provides medially directed reactive forces proximal and distal to the greater trochanter. In this arrangement the abduction angle of the femur in the residual limb more closely approaches that of the femur in the sound extremity during all portions of the user's gait, thus greatly reducing the characteristic limp. This recently developed socket design is not, however, designed by a universally accepted appellation. At present such a socket design is designated as a "N.S.N.A (Normal Shape and Normal Alignment) socket", or a "narrow ML (Medial/Lateral) socket" or a "Cat-Cam (Contoured, Adducted Trochanter, Controlled Alignment Method) socket".
Irrespective of the name employed, although it is believed that the term "Ischial Containment Socket" is most appropriate, in order to provide an effective socket which incorporates the narrow medial/lateral dimension a more precise fitting of the socket to each patient has heretofore been required, as are multiple tests of the socket prior to fabrication of the finished prosthetic limb. Because of the difficulty in achieving a functionally acceptable fit with this new design, more fitting time is required, and the patient must be willing to accept this inconvenience. Nevertheless, the greater comfort and the improved functionality that can be achieved by this new design has been thought to offset the inconvenience, and increased cost, at least to those who can afford both the time and expense.